1. Field of the Invention
The present invention relates to a heat-dissipating fan and, more particularly, to a small axial-flow fan that can prevent a rotor thereof from disengaging.
2. Description of the Related Art
The current trend of research and development in electronic products is miniaturization, and that leads to necessity of miniaturization of an axial-flow fan. For example, manufacturers replace a conventional retaining ring, which is disposed in a shaft tube to retain a shaft of a rotor, with a balancing plate that is made of magnetically conductive material and attracted by a permanent magnet fixed to the rotor, and then prevention of disengagement of the rotor is still achieved. Under stable operation, an axial thickness and an overall volume of a conventional heat-dissipating fan are reduced effectively to provide miniaturization of the conventional heat-dissipating fan, and thus the conventional heat-dissipating fan can be applied to a miniature electronic device or electronic apparatus.
FIG. 1 shows the above-mentioned conventional heat-dissipating fan 8, which has a balancing plate for displacing a retaining ring. The conventional heat-dissipating fan 8 includes a casing 81, a stator 82, a rotor 83 and the balancing plate 84. The casing 81 has a shaft tube 811 around which the stator 82 is mounted, and the rotor 83 has a shaft 831 rotatably inserted in the shaft tube 811 of the casing 81. The balancing plate 84 is mounted inside the casing 81 and attracted by a permanent magnet 832 that is fixed to the rotor 83, with a magnetically attracting spacing being formed between the balancing plate 84 and the permanent magnet 832. Thus, the rotor 83 disengaging from the casing 81 is avoided.
However, if the conventional heat-dissipating fan 8 is hit too hard while it is packed, moved, transported or operating, the rotor 83 will easily depart from the casing 81, because prevention of departure of the rotor 83 from the casing 81 is provided merely by the magnetic attraction between the balancing plate 84 and the permanent magnet 832.
Another conventional heat-dissipating fan is described in Taiwan Patent Issue No. M309021 entitled “MINIATURE FAN” and shown in FIG. 2. Referring to FIG. 2, the conventional heat-dissipating fan 9 includes an impeller 91, a frame 92 and a base 93. The impeller 91 has a shaft 911, and there is a permanent magnet 912 arranged around the impeller 91. The frame 92 has an inlet 921 and a guiding flange 922 formed on the periphery of the inlet 921 and extending towards the center of the inlet 921. The base 93 is coupled to the frame 92 and has a shaft tube 931 receiving the shaft 911 of the impeller 91. There is a magnetically positioning ring 932 mounted on the base 93 and facing the permanent magnet 912, so that the impeller 91 is prevented from disengaging from the frame 92 by magnetic attraction formed between the magnetically positioning ring 932 and the permanent magnet 912. Besides, by the configuration of the guiding flange 922, the impeller 91 is further retained inside the frame 92.
Nevertheless, the guiding flange 922 is in the form of a ring and formed on the periphery of the inlet 921 of the frame 92 and leads the conventional heat-dissipating fan 9 to have reduced air input and lowered heat-dissipating efficiency owing to a small diameter of the inlet 921. Also, the shaft tube 931, the magnetically positioning ring 932, the impeller 91 and other related components must be previously mounted to the base 93 before the frame 92 and the base 93 are assembled due to the arrangement of the guiding flange 922. Hence, the frame 92 and the base 93 can not be integrally formed to be a fan housing that is of merely one piece, so that inconvenience of assembling is provided.